This invention relates to a process for producing high purity methacrylic acid esters in increased yield compared to conventional processes. A number of commercial processes are practiced for the production of such esters including sulfuric acid treatment of acetone cyanohydrin, two stage oxidation of isobutylene or t-butyl alcohol, and liquid phase catalytic condensation of propionaldehyde with formaldehyde.
U.S. Pat. No. 4,529,816 describes a conventional acetone cyanohydrin (“ACH”) process for the production of methyl methacrylate (“MMA”) from ACH. In this process, ACH is hydrolyzed by sulfuric acid to produce α-hydroxyisobutyramide (“HIBAM”) and α-sulfatoisobutyramide (“SIBAM”). Next, the HIBAM and SIBAM are thermally converted to 2-methacrylamide (“MAM”) and a small amount of methacrylic acid (“MAA”). The MAM is esterified with methanol to produce the desired MMA product, while residual HIBAM is esterified to methyl α-hydroxyisobutyrate (“α-MOB”). The esterification product stream is a mixed product that is subjected to separation and purification steps to isolate the MMA product from the other compounds. Typically, a purified MMA product stream is produced, along with a purification residue comprising other compounds including, but not limited to, α-MOB and methyl β-methoxyisobutyrate (β-MEMOB). The recovery and conversion of one or more of these other compounds to produce additional MMA product has been the subject of various research and development efforts having varying degrees of success and practical utility. In particular, U.S. Pat. No. 4,529,816 describes an improvement wherein the α-MOB is isolated and recycled to the process between the thermal conversion and esterification steps.
U.S. Pat. No. 5,393,918 describes a process similar to that of U.S. Pat. No. 4,529,816, but the thermal conversion step is eliminated. Instead, the hydrolysis product is subjected directly to esterification with methanol to produce an esterification product that includes the desired MMA product, as well as α-MOB and β-MEMOB. In the process described in U.S. Pat. No. 5,393,918, the esterification (“crude MMA”) product is subjected to distillation to recover the product MMA and produces a liquid residue stream comprising α-MOB and β-MEMOB. The α-MOB and β-MEMOB are separated from the residue stream, typically by fractional distillation. The recovered α-MOB and β-MEMOB are subjected to vapor phase catalytic dehydration, using a crystalline aluminosilicate, to produce a recycle mixture comprising MMA, methacrylic acid (“MAA”), methanol and water, which is recycled to the process between the hydrolysis and esterification steps, or between the esterification and separation steps. The dehydration of α-MOB and β-MEMOB is performed in the vapor phase and in the presence of a crystalline aluminosilicate catalyst, which may or may not be promoted with an alkali metal or a platinum group element.
U.S. Pat. No. 5,087,736 discloses an ACH process that does not require sulfuric acid for preparing methacrylic acid esters. A key step of this process is the vapor phase dehydration of α-MOB in the presence of an alkali metal and platinum group element modified crystalline aluminosilicate, as disclosed in U.S. Pat. No. 5,068,399. However, this process suffers from low yields based on ACH.
FR 2064583 and GB 1256288 disclose purifications of a crude MMA product stream to produce a residual bottoms stream containing α-MOB and MAA, both of which are then converted to MMA by treatment with sulfuric acid and methanol, respectively and concurrently. The conversion of α-MOB and MAA is performed separate and apart from the hydrolysis and esterification reaction steps of the conventional MMA process and is followed by distillation to recover the MMA produced.
A variety of solid catalysts have been used for converting α-MOB and/or β-MEMOB into MMA and MAA in the vapor phase. For example, in Japanese Patent Publication Nos. 20611/1969, 20612/1969 and 15724/1970, a phosphate-based acid or salt deposited onto silica or silica-alumina was used. These technologies were plagued by the need for very high reaction temperatures, unacceptable levels of by-product methyl isobutyrate (MIB) formation, and fast deactivation by coke deposition. Crystalline aluminosilicates containing alkali or alkaline earth metals have been thoroughly studied for the conversion of α-MOB and β-MEMOB into MMA and MAA, as disclosed in U.S. Pat. No. 5,371,273, U.S. Pat. No. 5,393,918, and U.S. Pat. No. 5,739,379, as well as JP Application No. 65896/1990, U.S. Pat. No. 5,250,729 and EP 429,800 A2. The dehydration of α-MOB to MMA was commercialized by the Mitsubishi Gas Chemical Company in 1997 as a sulfuric acid-free ACH-based MMA process. The art shows that crystalline aluminosilicates such as zeolite NaX are well suited for α-MOB dehydration; however, they are limited in their ability to achieve simultaneous high yields on α-MOB and β-MEMOB and, therefore, have limited applicability for MMA residue yield recovery.
Catalysts containing Cs and silica gels have been explored for a number of reactions, including dehydrations, aldol condensations and Michael additions, but not conversion of α-MOB and/or β-MEMOB into MMA and MAA in the vapor phase. U.S. Pat. No. 4,841,060, U.S. Pat. No. 5,625,076, and U.S. Pat. No. 5,304,656, for example, disclose catalysts containing silicon and at least one element selected from the group consisting of alkali metals and alkaline earth metals for intramolecular dehydrations, such as the conversion of mercaptoalkanols to alkylene sulfides, alkanolamines to cyclic amines, N-(2-hydroxyethyl)-2-pyrrolidone to N-vinyl-2-pyrrolidone, and tertiary N-(2-hydroxyalkyl) carboxylic acid amide to tertiary N-alkenyl carboxylic acid amide. The substrates and reactions involved in these processes, however, differ chemically from dehydration and demethanolation of α-MOB and β-MEMOB, respectively, to MMA.